This application claims the priority of German Patent Document No. 101 04 607.3-41, filed Feb. 2, 2001, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a gas generating system for a fuel cell system as well as to a method of operating a gas generating system according to the preamble of the independent claims.
The various components of a gas generating system, such as the reformers, selective oxidation stages, catalytic burners, require a certain operating temperature for their function, for example, at least 200EC for reforming reactors. For the start of a gas generating system, the components have to be heated to the operating temperature. The heating by means of electric energy is not expedient because of the poor efficiency. In addition, the available electric energy (mainly in vehicles) may be too low for starting the system. Furthermore, the starting time should be as short as possible.
Japanese Patent Document JP 8-133701A teaches providing an excess of oxygen into the catalyst to provide rapid warming for the cold start of a selective oxidation catalyst, so that, in addition to carbon monoxide, hydrogen is also oxidized.
German Patent Document DE 19825772 A1 teaches rapidly preheating a vapor reformer at the start by means of a heating element and subsequently feeding oxygen, together with methanol and water vapor, to the reformer in order to immediately start conversion reactions. After the operating temperature has been reached, the oxygen supply may be stopped.
Finally, German Patent Document DE 3345958 A1 teaches burning methanol with air guiding the hot combustion waste gases for the heat exchange along the catalyst bed in order to subsequently guide them directly through the catalyst bed. When the operating temperature of 480EC has been reached, hydrogen of the fuel cell exhaust gas is used for the heating in order to maintain the required cracking temperature.
It is an object of the present invention to provide a method for a gas generating system for a fuel cell system to start the system as fast as possible, and to provide an appropriately further developed gas generating system. The fuel cell system can be used to supply electricity to any device that consumes electricity, such as the electric motor of a vehicle. In particular, during the starting phase, hydrogen-containing fuel should be made available to the fuel cell system as fast as possible and the exhaust gas emissions should be limited to an acceptable reliable amount.
This object is achieved by means of the embodiments of the invention described below.
In the method according to the invention, for starting the operation of the gas generating system, at least one of the catalytic burners is started and the heat generated by the burner is used for the evaporation of water and of the hydrocarbons (to be fed to the reformer), such as methanol, and for heating a partial reformer connected to the reformer. The partial reformer operates with a lower throughput than the actual reformer of the gas generating system. Instead of the partial reformer, a partial oxidation stage (POX) may be used to convert fed hydrocarbons, such as methanol by means of an oxidant, such as air, to form a hydrogen-containing gas. In this case, the heating of the components takes place essentially by way of the chemical reaction of combustion media so that, for starting the system, only limited electric energy from a vehicle battery is required.
As a result, the method according to the invention can bring at least some of the components of the gas generating system to the operating temperature within a short time. The partial reforming unit or partial oxidation unit supplies reformate or hydrogen-containing gas which is purified with respect to carbon monoxide in a selective oxidation stage connected on the output side. The operation of the fuel cell can be started by means of the purified hydrogen-containing gas. The fuel cell generates current so that a driving operation becomes possible in the case of a vehicle and the auxiliary aggregates of the fuel cell system, such as compressors, are now fed by the fuel cell and no longer by the electric auxiliary energy, such as the vehicle battery. The hot gases or waste gases of the already heated components continuously heat the remaining components. Another advantage is that, also during the cold start of the system, undesirable emissions can be prevented because the emissions can be converted by the catalytic burner or burners.
The invention is particularly suitable for vehicles which are equipped with a fuel cell system.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.